To meet the demand for wireless data traffic having increased since deployment of 4th-generation (4G) communication systems, efforts have been made to develop an improved 5th-generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post long term evolution (LTE) system’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like.
In the 5G system, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Vigorous research efforts are recently underway to support low latency in wireless communication systems. Latency may be defined using, e.g., one-trip time which signifies the time between transmission of data by a transmitter and reception of the data by a receiver. Reducing the performance index representing such latency may quicken real-time streaming, vehicle-to-X (V2X), or other communication services.
For the following reasons, latency may occur:
First, in the case of uplink transmission, a user equipment (UE) requests a base station to do scheduling. The base station allocates uplink (UL) resources to the UE, and the UE transmits data to the base station. Such 3-step UL transmission procedure may increase the time between when data to be transmitted from the UE is generated and when the data is received by the base station.
Second, current LTE systems have been designed based on the transmission time interval (TTI) being 1 ms. The TTI of 1 ms means that the interval between two consecutive schedulings is 1 ms and that a base station or UE should wait up to 1 ms in order to transmit data. Such wait time is a cause of increasing the overall latency in the wireless communication system.
Third, a base station or UE, after receiving data, should conduct processing, such as encoding or decoding, on the received data. The processing time is another cause of increasing the overall latency in the wireless communication system.
Finally, when a receiver fails to receive data, a retransmission process, such as hybrid automatic repeat request (HARQ), is carried out. The retransmission process brings about acknowledgement (ACK)/negative ACK (NACK) feedback for determining whether data reception succeeds or fails, and failure to receive causes a delay due to the processing and retransmission of data already sent. In an LTE system, the transmitter chooses a proper modulation and coding scheme (MCS) utilizing a channel quality indicator (CQI) feedback or sounding reference signal transmitted from the receiver and transmits data by the chosen MCS. The receiver's failure to receive the data transmitted by the chosen MCS is typically attributed to a CQI mismatch. A CQI mismatch arises when the signal to interference plus noise ratio (SINR) and MCS when the transmitter generates data differ from the SINR and MCS of a link formed when the transmitter transmits data.
Meanwhile, LTE systems running on unlicensed bands are being researched. Such CQI mismatch may be drastically increased in LTE systems operated along with Wi-Fi access points (APs), on unlicensed bands. This is why the Wi-Fi APs, as hidden nodes, irregularly working in light of geographical location and time, may interfere with base stations or UEs in the LTE systems.
FIGS. 1A and 1B illustrate a HARQ operation performed in a typical wireless communication system according to the related art.
Referring to FIG. 1A, when a transmitter, after receiving a CQI feedback from a receiver, initially transmits data in an nth subframe according to an MCS selected corresponding to the CQI feedback (101), the receiver determines whether data is successfully received, and if so, feeds back an ACK in an n+4th subframe (103). When receiving the ACK, the transmitter transmits new data in an n+8th subframe (105).
Referring to FIG. 1B, when a transmitter, after receiving a CQI feedback from a receiver, initially transmits data in an nth subframe according to an MCS selected corresponding to the CQI feedback (111), the receiver determines whether data is successfully received, and if not, feeds back a NACK in an n+4th subframe (113). When receiving the NACK, the transmitter retransmits the data initially transmitted in the n+8th subframe (115). Accordingly, a HARQ round-trip time occurring in the HARQ operation shown in FIGS. 1A and 1B may be regarded as 8 ms because one TTI is 1 ms.
As such, a need exists for a scheme for reducing latency occurring in the general HARQ operation, and such latency may be more frequent in LTE systems operating together with Wi-Fi APs on unlicensed bands.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.